First of all, I have not read all the replies. My apologies if I am writing something that has already been told.

If your budget is low and you are just starting, I'd suggest canon. The reason for this is, canon's lower end lenses (even the mid range lenses like 300 f4) perform better than nikon counterparts. If you are talking of lenses of the class of a 300 2.8 or 500 f4, then it is another story. In that case, it will boil down to one's personal preference and comfort levels with the system. In fact, a few months ago, I too was in a dilemma of choosing between the canon 300 f/2.8 L IS II USM/ 500 f/4 L IS II USM and nikon ED 200-400 f/4 VR II or 500 f/4 ED VR II . I decided to get the canon because of its lighter weight and hence more easier to lug around when travelling.

Cheers and happy buying.